vue2源码系列-响应式原理

export function observe (value: any, asRootData: ?boolean): Observer | void {
  if (!isObject(value) || value instanceof VNode) {
    return
  }
  // 如果已经监听数据则直接返回
  let ob: Observer | void
  if (hasOwn(value, '__ob__') && value.__ob__ instanceof Observer) {
    ob = value.__ob__
  } else if (
    shouldObserve &&
    !isServerRendering() &&
    (Array.isArray(value) || isPlainObject(value)) &&
    Object.isExtensible(value) &&
    !value._isVue
  ) {
    // 实例化监听者
    ob = new Observer(value)
  }
  if (asRootData && ob) {
    ob.vmCount++
  }
  return ob
}
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export class Observer {
  value: any;
  dep: Dep;
  vmCount: number; // number of vms that have this object as root $data
  constructor (value: any) {
    this.value = value
    this.dep = new Dep()
    this.vmCount = 0
    // 定义__ob__属性
    def(value, '__ob__', this)
    // 数组处理
    if (Array.isArray(value)) {
      // 我们知道Object.defineProperty无法监听数组的改变
      // 所以这边其实是重写了数组方法，使其支持监听，例如splice
      if (hasProto) {
        protoAugment(value, arrayMethods)
      } else {
        copyAugment(value, arrayMethods, arrayKeys)
      }
      this.observeArray(value)
    } else {
      this.walk(value)
    }
  }
  walk (obj: Object) {
    // 遍历属性
    const keys = Object.keys(obj)
    for (let i = 0; i < keys.length; i++) {
      defineReactive(obj, keys[i])
    }
  }
  observeArray (items: Array<any>) {
    for (let i = 0, l = items.length; i < l; i++) {
      observe(items[i])
    }
  }
}
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export function defineReactive (
  obj: Object,
  key: string,
  val: any,
  customSetter?: ?Function,
  shallow?: boolean
) {
  // 每个属性会有一个dep实例
  const dep = new Dep()
  const property = Object.getOwnPropertyDescriptor(obj, key)
  if (property && property.configurable === false) {
    return
  }
  // 这里会存着属性自身的 set get 函数
  // cater for pre-defined getter/setters
  const getter = property && property.get
  const setter = property && property.set
  if ((!getter || setter) && arguments.length === 2) {
    val = obj[key]
  }
  // 递归子属性进行数据监听
  let childOb = !shallow && observe(val)
  // 重点：数据劫持
  Object.defineProperty(obj, key, {
    enumerable: true,
    configurable: true,
    get: function reactiveGetter () {
      // 如有开发者自定义的get则执行
      const value = getter ? getter.call(obj) : val
      // Dep.target作为连接dep和watcher的纽带
      if (Dep.target) {
        // dep添加订阅者Watcher实例
        // Watcher实例的来源后面会说到
        dep.depend()
        // 未知作用：暂不讲解
        if (childOb) {
          childOb.dep.depend()
          if (Array.isArray(value)) {
            dependArray(value)
          }
        }
      }
      return value
    },
    set: function reactiveSetter (newVal) {
      // 如有开发者自定义的get则执行，注意执行的也是get，因为这里是取值
      const value = getter ? getter.call(obj) : val
      // 判断数据更新 (newVal !== newVal && value !== value) 暂不清楚是干嘛的 猜测是处理某些情况导致数据修改的异常
      /* eslint-disable no-self-compare */
      if (newVal === value || (newVal !== newVal && value !== value)) {
        return
      }
      /* eslint-enable no-self-compare */
      if (process.env.NODE_ENV !== 'production' && customSetter) {
        customSetter()
      }
      // #7981: for accessor properties without setter
      if (getter && !setter) return
      if (setter) {
      // 如有开发者自定义的set则执行
        setter.call(obj, newVal)
      } else {
        // 设置新值
        val = newVal
      }
      // 对新值也要进行监听
      childOb = !shallow && observe(newVal)
      // dep进行发布通知 这里将在get中收集的watcher逐一进行通知
      dep.notify()
    }
  })
}
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let uid = 0
export default class Dep {
  static target: ?Watcher;
  id: number;
  subs: Array<Watcher>;
  constructor () {
    this.id = uid++
    // 订阅者列表
    this.subs = []
  }
  // 添加订阅者
  addSub (sub: Watcher) {
    this.subs.push(sub)
  }
  removeSub (sub: Watcher) {
    remove(this.subs, sub)
  }
  // 将当前watcher实例作为订阅者添加
  depend () {
    if (Dep.target) {
      Dep.target.addDep(this)
    }
  }
  notify () {
    const subs = this.subs.slice()
    if (process.env.NODE_ENV !== 'production' && !config.async) {
      subs.sort((a, b) => a.id - b.id)
    }
    // 通知订阅者更新
    for (let i = 0, l = subs.length; i < l; i++) {
      subs[i].update()
    }
  }
}
// 缓存watcher实例
// 同一时间仅有一个正在实例化的watcher
Dep.target = null
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let uid = 0
export default class Watcher {
  vm: Component;
  expression: string;
  cb: Function;
  id: number;
  deep: boolean;
  user: boolean;
  lazy: boolean;
  sync: boolean;
  dirty: boolean;
  active: boolean;
  deps: Array<Dep>;
  newDeps: Array<Dep>;
  depIds: SimpleSet;
  newDepIds: SimpleSet;
  before: ?Function;
  getter: Function;
  value: any;
  constructor (
    vm: Component,
    expOrFn: string | Function,
    cb: Function,
    options?: ?Object,
    isRenderWatcher?: boolean
  ) {
    this.vm = vm
    // 顾名思义渲染watch
    if (isRenderWatcher) {
      vm._watcher = this
    }
    // _watchers添加当前实例
    vm._watchers.push(this)
    // options
    if (options) {
      this.deep = !!options.deep
      this.user = !!options.user
      this.lazy = !!options.lazy
      this.sync = !!options.sync
      this.before = options.before
    } else {
      this.deep = this.user = this.lazy = this.sync = false
    }
    // 初始化配置属性 暂时跳过
    this.cb = cb
    this.id = ++uid // uid for batching
    this.active = true
    this.dirty = this.lazy // for lazy watchers
    this.deps = []
    this.newDeps = []
    this.depIds = new Set()
    this.newDepIds = new Set()
    this.expression = process.env.NODE_ENV !== 'production'
      ? expOrFn.toString()
      : ''
    // parse expression for getter
    if (typeof expOrFn === 'function') {
      // expOrFn是函数
      this.getter = expOrFn
    } else {
      // expOrFn是表达式则转化为函数
      this.getter = parsePath(expOrFn)
      if (!this.getter) {
        this.getter = noop
      }
    }
    // 执行get
    this.value = this.lazy
      ? undefined
      : this.get()
  }
  get () {
    // 将静态属性设置为当前watcher实例Dep.target = this
    pushTarget(this)
    let value
    const vm = this.vm
    try {
      // 执行getter也就是expOrFn
      // expOrFn函数将访问到我们的vm数据，所以将触发我们在defineReactive函数定义的get函数
      // 此处便解答了defineReactive中Dep.target的来源
      // new Watcher -> 执行get -> Dep.target = this -> 执行expOrFn -> 触发数据get函数 -> dep.depend() -> 属性的dep添加订阅者（this）
      // 这样就完成了数据属性的订阅
      value = this.getter.call(vm, vm)
    } catch (e) {
      if (this.user) {
        handleError(e, vm, `getter for watcher "${this.expression}"`)
      } else {
        throw e
      }
    } finally {
      // 深度watcher 暂时跳过
      if (this.deep) {
        traverse(value)
      }
      popTarget()
      this.cleanupDeps()
    }
    return value
  }
  addDep (dep: Dep) {
    // 在newDeps列表中存储dep
    const id = dep.id
    if (!this.newDepIds.has(id)) {
      this.newDepIds.add(id)
      this.newDeps.push(dep)
      if (!this.depIds.has(id)) {
        dep.addSub(this)
      }
    }
  }
  // 清除dep
  cleanupDeps () {
    let i = this.deps.length
    // 仅存在deps中的dep清除订阅
    while (i--) {
      const dep = this.deps[i]
      if (!this.newDepIds.has(dep.id)) {
        dep.removeSub(this)
      }
    }
    // 清空depIds
    // 这边使用了两个数组交换 我猜测是为了节约内存 如果单纯设置 newDepIds = [] 的话需要新开辟内存
    let tmp = this.depIds
    this.depIds = this.newDepIds
    this.newDepIds = tmp
    this.newDepIds.clear()
    tmp = this.deps
    this.deps = this.newDeps
    this.newDeps = tmp
    this.newDeps.length = 0
  }
  // 数据更新
  update () {
    if (this.lazy) {
      this.dirty = true
    } else if (this.sync) {
      // 同步watcher
      this.run()
    } else {
      // 异步watcher
      queueWatcher(this)
    }
  }
  // 执行回调
  run () {
    if (this.active) {
      const value = this.get()
      if (
        // 判断新旧值是否有改变
        value !== this.value ||
        // Deep watchers and watchers on Object/Arrays should fire even
        // when the value is the same, because the value may
        // have mutated.
        isObject(value) ||
        this.deep
      ) {
        // set new value
        const oldValue = this.value
        this.value = value
        if (this.user) {
          const info = `callback for watcher "${this.expression}"`
          invokeWithErrorHandling(this.cb, this.vm, [value, oldValue], this.vm, info)
        } else {
          // 执行回调
          this.cb.call(this.vm, value, oldValue)
        }
      }
    }
  }
  // ...
}
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